The generally accepted method for accurately measuring power frequency AC currents uses a current transformer. A current transformer is generally a toroid-shaped magnetic core with windings around the core, typically about 100 turns or more. The magnetic core is constructed of some ferrous material or alloy. In use, the conductor, the current in which is to be measured is passed through the hole in the center of the toroidal magnetic core. The current flowing in the conductor to be measured generates a magnetic field, which is captured within the magnetic toroid of the current transformer which surrounds it. This magnetic field induces a current in the windings around the toroidal core in the ratio of the number of windings to one, the one being the conductor passing once through the core of the toroid. With 100 turns around the core, this current transformer has a ratio of 100:1. The advantages of this type of current transformer are several: high accuracy, rejection of or insensitivity to current carrying conductors anywhere outside the core and very low sensitivity as to how the conductor to be measured is placed or positioned through the hole in the center of the magnetic core.
The primary disadvantage of using a toroidal magnetic iron core current transformer in pre-existing circuits is that it cannot simply be placed around the conductor to be measured. For this reason, current transformers of this type are generally restricted to laboratory use. Iron core current transformers can, however, be constructed with hinged openings in the iron core so as to allow the core to be temporarily opened while it is placed over a conductor to be measured and these types of devices do have good accuracy and good rejection of current-carrying conductors outside the center of the core. As a practical matter however, the mechanical complexity of opening and closing the hinged core and the weight of the iron core are substantial disadvantages for a tool to be used in the field.
There are alternative current transformer designs which do not use any heavy magnetic core and the Rogowski coil is the most popular. However, for best accuracy, the Rogowski coil must also be constructed so as to completely encircle the conductor to be measured. Rogowski coils provided with a permanent opening so as to allow the coil to be placed conveniently over or around the conductor to be measured suffer from reduced accuracy and consistency, in that the measured current can vary with the position of the conductor to be measured relative to the opening in the Rogowski coil. In addition, other current carrying conductors placed nearby, but still outside of the opening of the Rogowski coil, will influence the measurements made of conductors inside the opening.
Two noteworthy improvements have been made to the open Rogowski coil. One is described in U.S. Pat. No. 5,057,769 and discloses compensating coils located at the opening in the Rogowski coil. This approach works well to allow accurate measurements to be made for conductors inside the opening, but it does not reduce the influence of conductors just outside the opening.
A second improvement is described in U.S. Pat. No. 6,717,397, which discloses a modification of the Rogowski coil to replace the continuous coil with many small discrete coils with an equal number of openings between them. While this construction allows great flexibility in design and low-cost construction, it has considerable disadvantages in terms of how the conductor to be measured must be centered with respect to the cluster of individual coils, and it still allows substantial influence of conductors outside of the cluster of individual coils.
While these two improvements do allow for convenience of use and for compact and lightweight construction, neither of them equals the performance of a closed Rogowski coil or of an iron core current transformer, with their accurate measurement of conductors inside the core/coil and high degree of rejection of conductors outside the core/coil.
A number of companies presently manufacture high-voltage “hot-stick” ammeters designed to measure power frequency line current by direct application to power lines, which may be energized at voltages up to 69 kV or more. For example, referring to FIG. 1, such an ammeter 10 is shown, provided with a sensing head 11 adapted to fit around an electric power line 12, which may, e.g., may be a power transmission or distribution line, carried by utility poles 13 or the like. Such ammeters are often used with an extension hot-stick 15, which both extends the reach of the user and insulates the user from the high voltage which may be present on the power line.
Hot-stick ammeters are generally used to make instantaneous or very short-term measurements of current flowing in a power line. In use they are placed around a power line for only a few seconds, during which the current flowing in the power is measured by means of some type of current sensor or current transformer, after which the ammeter is withdrawn from the power line and the measured current can be viewed in a display, which may be digital.
For ease of use, the sensing head 11 of the high-voltage hot-stick ammeter 10 is designed as a simple open channel in a U or C shape, so that it can be easily placed over or around the power line 12. When the device is placed over the power line 12, as shown, the strength of the magnetic field from the power line, which is directly proportional to the current flowing in the line, is detected and sensed by the current sensing portion of the ammeter. Suitable sensing coils and electronic circuits measure the strength of the magnetic field and convert it to digital form to be displayed as digits on the front panel of the ammeter and/or to be recorded in digital form within the ammeter for later display or transfer to other devices. The displayed or recorded digits indicate amperes of current flowing in the power line.
Such ammeters suffer from the disadvantages discussed above, with respect to Rogowski coils having a permanent opening. Furthermore, in hot-stick applications, wherein the user may be positioned at some distance from the conductor or power line, the current of which is to be measured, it is frequently difficult to tell whether or not the conductor is positioned relative to the sensing head so as to ensure accurate current measurement. Thus, the user may not be sure whether or not the displayed current reading is an accurate reading.